The present invention relates generally to a device for transfer between structures and more specifically, to a device for transfer between a substantially static structure and a fully dynamic structure, such as between an offshore platform and a boat.
Several methods have been and are now being used to effectuate transfer between static structures, such as platforms or docks, and fully dynamic structures, such as boats. One such method is a swing rope in which the person to be transferred grasps onto a rope which is tied onto the static structure, and swings from one structure to the other over a gap of water. This maneuver, however, becomes much more difficult to perform as the roughness of the water increases. The resulting acrobatic difficulty of performing this maneuver in rough seas, as well as the necessity of an increased gap between the boat and static structure, may cause this maneuver to become very dangerous and/or impossible to perform. This is because the person attempting to swing on the rope is being moved with the same magnitude and in the same directions as the dynamic structure is moving, and it is much more difficult for a person to judge changes in movement of an object moving toward and away from them, than it is for an object moving laterally across the person's vision.
Also, when the person is attempting to transfer from the dynamic structure to the static structure, the person needs to grasp onto the swing rope at such point in time that the dynamic structure is at its highest elevation, otherwise, the person would be dragged by the dynamic structure. When the person grasps onto the swing rope, the person becomes a human pendulum, because they are free to swing in a vertical plane under the influence of gravitational force only. The length of such human pendulum is determined by the elevation of the dynamic structure when the person decided to grasp onto the swing rope. This length remains constant during the swing(s), and might not properly fit the fixed target of the static structure, causing the person to swing too high or too low.
When the person is attempting to transfer from the static structure to the dynamic structure, the length of the human pendulum (length of the swing rope) is fixed because of the fixed geometry of the static structure. However, the person is swinging to a randomly moving target, and the fixed length of the swing rope might become too short or too long for that specific point in time.
A second means of transfer is the use of a rope ladder suspended from a beam installed on a static structure. By means of a mechanical device, such as a rope, a beam is first forced to rotate to a certain point above the surface of the dynamic structure, such as a vessel's deck. The person being transferred climbs up on the rope ladder to a certain elevation, and the beam is rotated back to the platform. The person then descends from the rope ladder to the static structure, or vice versa. This method is limited first in the fact that the length of the beam is limited by structural considerations. Because the beam can only have a certain length based on the available space on the static structure, the width of the gap of water between the vessel and the platform during rough sea conditions may exceed the maximum length of the beam. This would preclude transfer in these conditions. Also, transferring personnel in this manner depends heavily on the acrobatic skills of the person being transferred. The person needs to be synchronized with the descending-ascending movement of the rope ladder relative to the vessel's deck, resulting from the random motion of the waves. If the person grips the rope ladder at the wrong time, or does not climb fast enough, it is possible that the person can be hit by the vessel's deck during the crest of the wave. Because a person's perception of movements relative to his frame of reference has better resolution to movements contained in a plane normal to his line of sight, rather than movements parallel to his line of sight, a person being transferred may have problems when descending the rope ladder onto the vessel's deck. The person does not have an adequate feedback on how long, how fast, and when his last step to reach the vessel's deck should be. This frequently results in either premature or late last steps, causing the person to fall onto the deck of the vessel.
Another method of transfer is the use of a basket which can be lowered onto a vessel and then lifted. In this method, a crane, which is located on the static structure, can descend a basket onto a vessel's deck and goods or personnel can be loaded onto the basket and lifted onto the structure. This method, which sounds relatively simple, can become very difficult in rough seas where a vessel's deck is moving in many directions at once. Also, if the static structure is unmanned, as is the case with many offshore platforms, this method of transfer is not available for the crane's operator.
A fully floating bridge has also been used to effectuate transfer over water. However, because of the evenly distributed buoyancy of the bridge and its inherent flexibility, the magnitude of movement of the bridge's surface is the same as the magnitude of the waves on the surface. Also, any pitch, roll, horizontal or vertical movement on the buoyant part of the bridge is directly translated to the bridge's surface. Therefore, while transfer of this type may be relatively easy in calm seas, it can be very difficult and perhaps impossible to perform in rough seas.
The transfer between the end of the floating bridge and the vessel's deck also depends heavily on the acrobatic skills of the person being transferred. The person needs to match the randomized and unsynchronized movements of both the bridge and vessel, each having different and independent bouyancies.
Many industries, such as the oil and gas industry, are dependent on offshore operation, and it is very important that personnel and equipment be able to be transferred from a dynamic structure floating on water, such as a boat, and a static structure, such as a dock or offshore platform. Therefore, there is a need for an improved coupling device that can be used effectively in both rough and normal seas, and which adapts to the six degrees of freedom of a dynamic structure and converts them into the smaller horizontal movements of a bridge relative to that dynamic structure. This needs to be done while maintaining stable physical contact between the dynamic structure and the static structure.